Most laser gyros commercially available at the present time consist of a cavity made of zerodur containing a helium/neon gas mixture, in which cavity two counterpropagating optical waves coexist, i.e. waves propagating in opposite directions inside the cavity. It is known that in this type of laser, when the rotation velocities are low, the two counterpropagating waves have the same frequency. This frequency-locking problem at low rotation velocities or “dead zone” problem is usually solved by making the cavity vibrate about its rotation axis by mechanical dithering. A rotation velocity sufficient to prevent frequency locking is thus artificially created. This technique is however a non-inconsiderable source of noise because of the “random walk” phenomenon corresponding to the accumulation of a random phase at each pass through the dead zone.
To eliminate these drawbacks, another type of laser gyro has been developed. The physical principal consists in making not two but four waves coexist in the cavity, corresponding to two orthogonal polarization states and two opposite propagation directions. The patent by K. Andringa with reference U.S. Pat. No. 3,854,819 describes a device of this kind. This type of laser, also called a “multi-oscillator laser gyro”, associated with a magneto-optic frequency bias, makes it possible to eliminate the “dead zone” effect over the entire operating range of the laser gyro while still obviating, by an astute recombination of the four modes present in the laser cavity, fluctuations of said magneto-optic bias, which constitute without this a source with too large a drift for most laser gyro applications.
It is also possible to produce laser gyros with solid-state amplifying media, for example a laser-diode-pumped Nd:YAG crystal. The publication by S. Schwartz, G. Feugnet, P. Bouyer, E. Lariontsev, A. Aspect and J.-P. Pocholle published in Phys. Rev. Lett. 97, 093902 (2006) describes a laser gyro of this type.
In these devices, the competition between modes is no longer counterbalanced by the Doppler effect, as is the case in helium-neon gas laser gyros, but by an additional stabilizing device, for example the use of a feedback loop creating differential losses proportional to the difference in intensity between the counterpropagating modes of the laser, this being referred to as “active stabilization”. Such a device, although it proves to be relatively simple to implement for a solid-state laser gyro of the “two wave” type as described in the patent by S. Schwartz, G. Feugnet et J.-P. Pocholle with the reference U.S. Pat. No. 7,548,572, is however more complicated to implement in its “four wave” or multi-oscillator version as described in the patent by S. Schwartz, G. Feugnet and J.-P. Pocholle with the reference U.S. Pat. No. 7,230,686. FIG. 1 shows a laser gyro of this type, which essentially comprises:                an optical ring cavity 1 comprising at least one partially reflecting mirror 11, enabling the counterpropagating modes outside the cavity 1 to be treated;        a solid-state amplifying medium 2;        a slaving device 3 controlling the optical rotator or rotators 4 and 5 (dotted arrows in FIG. 1);        a measurement device 6;        an optical system comprising:                    a first optical assembly consisting of a first non-reciprocal optical rotator 5 and of a reciprocal optical rotator 4;            a second optical assembly consisting of a first spatial filtering device 7 and of a first polarization-splitting optical element 8;            a third optical assembly consisting of a second spatial filtering device 10 and of a second polarization-splitting optical element 9, the second optical assembly and the third optical assembly being placed on either side of the first optical assembly, the third optical assembly being placed symmetrically with respect to the second optical assembly; and            a fourth optical assembly consisting, in succession, of a first quarter-wave plate 12, a second non-reciprocal optical rotator 14 and a second quarter-wave plate 13, the principal axes of which are perpendicular at 90° to those of the first quarter-wave plate.                        